Engine gaskets are important parts of automotive engines. The gaskets are positioned between a cylinder head and an engine block which define a combustion chamber of an automotive engine. As shown in FIGS. 1(a) and 1(b), an engine gasket 1 is a sealing member having an opening 2, which generally has a circular shape with the same diameter as the cylinder of the engine, and an annular bead 3 which is a ridge formed by beading so as to surround the opening. The bead 3 functions as a seal since it is compressed between the cylinder head and engine block and blocks the interstice therebetween to prevent leakage of combustion gas, cooling water, and lubricating oil from the combustion chamber.
A material for fabricating such a gasket is, therefore, required to have high strength (high hardness) sufficient to retain a bead against compression, along with good workability and good corrosion resistance.
In order to meet the above-described requirements, a metastable austenitic stainless steel, such as SUS 301 stainless steel which is a Cr- and Ni-added stainless steel, has been used to fabricate engine gaskets. Deformation of such a steel by cold working, such as cold rolling and beading, causes the metastable austenite in the deformed area to transform to martensite which has a greater hardness. Thus, the steel can exhibit a high work hardenability with good workability.
However, such a stainless steel has the disadvantage that its properties, particularly hardness may fluctuate greatly, since the increased hardness of the steel obtained by working may vary significantly depending on the working ratio of the steel and the temperature at which the steel is subjected to working. Therefore, the quality, particularly sealing quality of gaskets made from the steel may fluctuate significantly. Another disadvantage is that the metastable austenitic steel is susceptible to stress corrosion cracking. Furthermore, the steel contains a large amount of nickel, which is expensive, thereby adding to the production costs of the gaskets.
In order to cope with these problems, a Cr-based martensitic stainless steel having a tempered martensitic structure has been proposed for the fabrication of engine gaskets in Japanese Patent Application Laid-Open No. 7-278758(1995). In general, martensitic stainless steel has improved resistance to stress corrosion cracking over the above-described metastable austenitic stainless steel. Moreover, it is relatively easy to achieve a high hardness with martensitic stainless steel by means of quenching, which causes transformation to form hard martensitic phases. Furthermore, martensitic steel is less expensive since it contains a very limited amount of expensive Ni.
However, since martensitic stainless steel as quenched has a decreased elongation and is difficult to work, it is essential that the quenched martensitic steel be subjected to heat treatment for tempering after quenching. Such heat treatment adds to the production costs of the steel and may cause embrittlement of the steel due to deposition of carbides or a loss of corrosion resistance due to the formation of Cr-deficient phases resulting from the deposition of carbides.
U.S. Pat. No. 5,624,504 discloses a martensitic-ferritic duplex-phase stainless steel which contains C, Si, Mn, P, S, Ni, Cr, N, B and Cu as essential alloying elements. The fraction of the martensite in the steel structure is selected so as to provide the steel with high strength, and the grain size of the martensite is as small as 10 .mu.m or less to assure good workability. The steel has a low carbon content of up to 0.10% by weight. This patent does not teach that the steel is suitable for use in the fabrication of gaskets.
Thus, there is a need for a high-performance, less expensive stainless steel for engine gaskets which can be produced in a stable manner.
These and other objects and advantages of the present invention will be apparent from the description as set forth below.